This invention is related to an earlier invention entitled "Dual Frequency Surface Coils" and assigned to the assignee of this invention. The prior invention was described in a patent application filed in the U.S. on Mar. 19, 1985 which received Ser. No. 713,689 U.S. Pat. No. 4,691,163 issued on the noted application on Sept. 1, 1987.
In MR data acquisition systems, radio frequency (RF) coils or probes are used to transmit RF pulses which tip or perturb nuclei ("spins") that have been aligned by static magnetic fields. After the RF pulses are removed, the perturbed nuclei tend to dephase and to subsequently return to their former aligned positions. When RF pulses of proper magnitude and frequency are applied and certain gradient pulses or combinations of RF and gradient pulses are applied the nuclei are rephased and RF echo signals are generated. These signals are detected by RF coils and provide data used to generate display images, for example. The most commonly perturbed element in magnetic resonance imaging (MRI) is hydrogen. Other elements are also perturbed, such as, for example, carbon. Also for a long time it has been known that phosphor can provide unique information. Thus, phosphor has been perturbed to obtain magnetic resonance MR spectroscopic data. For example, it is believed that there may be an abnormal abundance of phosphate in tissue that is tumorous. Accordingly, during some MRI procedures it is useful to obtain MR data based on the presence of phosphor in the patient.
Surface coils are special RF probes that are designed to operate when juxtaposed to a portion of the patient. The proximity improves the effect of the RF signal. For example, surface coils are used to obtain images of a female patient's breast using the echo signals of hydrogen nuclei. By using another frequency, the phosphor nuclei are perturbed and spectroscopic data is obtained showing whether there is an abnormal accumulation of phosphor in the breast. If such an accumulation is found it may be a reliable, early indication that a cancerous condition exists. This indication is obtainable long before other known tests for cancer can provide reliable data. The early detection of cancer is necessary for effecting a cure. Thus, it is extremely important to obtain such information as soon as possible.
In the past, it has been possible to first obtain the MRI data using one RF surface coil or probe tuned for the Larmor frequency of hydrogen and subsequently obtaining phosphor spectrometric data using another RF probe tuned for the Larmor frequency of phosphor. The different coils were used since different frequencies are necessary to perturb the different nuclei. It has not been common to use the same coil for the different frequencies because, among other things, of the difficulties in matching the impedance of the same probe to the characteristic impedences of the generator and receiver at different frequencies.
However, the use of the same coil has indeed been suggested for radio frequency probe operating at different frequencies in nuclear magnetic resonance. For example, U.S. Pat. No. 4,446,431 teaches a single coil for use at two frequencies. The tuning is accomplished remotely by using a transmission line wherein the high frequency signal is coupled at one point and the low frequency signal is coupled at another point on the transmission line.
The junction point for the low frequency signalin the single sample coil of the patent is extremely critical, as is the transmission line length. Another drawback of the double tuned single coil probe of the noted patent is that it has to be separately tuned for each sample being nutated or perturbed. When the probe is used for acquiring data of inanimate objects this presents more of an inconvenience than a problem. The inconvenience is largely due to the increase in throughput time. However, when the sample is a human being then the retuning for each patient is not only time consuming for the clinic which decreases throughput, but is also time consuming for the patient and detrimental to the patient's comfort.
The prior art also teaches the use of wide band probe arrangements for MR spectrometers. See for example U.S. Pat. Nos. 4,075,552 and 4,129,822. Both patents teach tunable tank or trap circuits that are individually tuned to obtain the MR frequencies of samples. The coil and associated circuitry are designed to match the impedances of both the transmitter and receiver at a wide range of frequencies. The U.S. Pat. No. 4,075,552 uses an autotransformer as the inductance of the tank circuit while the U.S. Pat. No. 4,129,822 uses a plurality of directly connected coils, in a series parallel arrangement with a switching arrangements for inserting and removing a coil for different frequency ranges.
The balanced surface coil of U.S. Pat. No. 4,691,163 represents an advancement in the art because, among other things, it can be used at two different frequencies without requiring separate tuning for each of the frequencies and it is not adversely loaded by the samples under test. Thus unlike other probes, the probe of that application does not require separate tuning for each sample or each change of distance between the sample and the coil.
The surface coil of application Ser. No. 713,689 required at least one pair of shielded cables. The cables of the pair of shielded cables are each connected to a different terminal of the balanced surface probe and the different frequency signals are both coupled to and from the probe at the same terminals by the single pair of shielded cables.
More recent prior art teaches multiple tuned probes using a separate trap circuit in series with the capacitor tuned sample coil for every desired resonant frequency. See for example an article entitled "A New Double-Tuned Probe for Concurrent 'H and "P NMR" by M. D. Schnall et al in the Journal of Magnetic Resonance, Vol. 65, pp. 122-129 (1985).
The noted prior art, including the U.S. patent application referred to hereinbefore in effect, teach either:
(1) Single coils or directly coupled coils with the resonant frequency being varied by variable capacitors, or inductors or combinations thereof;
(2) Circuits resonant at different frequencies depending on where the input/output leads are connected; or
(3) The use of trap circuits to insure resonance at desired frequencies.
The prior art devices each suffer from some major defects. For example:
(1) In some prior art surface probes, it is necessary to tune the circuit each time the frequency is changed from high to low or vice-versa and/or either single or unbalanced coils are used in the RF probes. With probes made with unbalanced coils it may be necessary to tune the probe for each pateint. Periodic tuning operations may not be sufficient to set the tuning for a plurality of specimens or samples (patients);
(2) In some prior art surface probes, multiple shielded cables are necessary to connect each of the resonant frequencies going to and from the probe. This hampers the surface coils in addition to adding additional shielding cabling,
(3) In some prior art surface probes, the efficiency of the probes at the higher frequency ('H) is less than 50%. Since the received signals in MR systems are inherently weak everything must be done to increase the efficiency of each part of the transmission and receiving systems, or
(4) In some prior art surface probes, the optimum power transfer to and from the probe does not occur at each resonance frequency of the probe.
Accordingly, there is a need for surface probes for use in MR operations that can be used at a plurality of frequencies, without requiring separate tuning for each of the frequencies, without requiring separate shielded cables for each of the frequencies, which are efficiently matched to the basic line impedance for all of the frequencies and which perform efficiently and with a minimum of reactive components.
Therefore, it is an object of the present invention to provide radio frequency balanced probes for magnetic resonance operations which are tuned to a plurality of frequencies so that the probe can be used for acquiring MR data of different elements (hydrogen, carbon and/or phosphor nuclei, for example) and which transmit and receive signals and/or pulses at the various frequencies without the necessity of removing and/or retuning the probes between the uses of the different frequencies and which have a single terminal pair and use only a single shielded cable for connection to the MR system. Ideally the RF probe provided also will not require tuning when used on different patients.